A large family of odorant receptor genes has been isolated based on its predicted homology to the class of seven transmembrane domain receptors. Individual sensory neurons of the olfactory system express wither one or a few odorant receptor subtypes, suggesting that the brain identifies the activation of specific odorant receptors by identifying activity from specific cells. Anatomical studies indicate that olfactory neurons expressing specific receptors are scattered randomly throughout the olfactory epithelium. Information from these dispersed cells may be integrated at discrete structures in the olfactory bulb, termed glomeruli, by the spatial segregation of sensory projections. The small receptor repertoire and experimental accessibility of the catfish make it an excellent system to study the organization of the olfactory system. The following experimental approaches are proposed: 1. To determine if only one, or alternatively, multiple receptor subtypes are expressed in a sensory neuron by double label in situ hybridization and single-cell polymerase chain reaction. The results from these experiments will reveal the complexity of the mechanisms used to identify the activation of specific odorant receptors and thereby decode olfactory information. 2. To determine the topography of the projections of neurons expressing the same receptor by a combination of fluorescent retrograde tracer injections into the olfactory bulb and in situ hybridizations with receptor probes. These experiments will determine whether spatial patterns of connectivity are used to encode olfactory information and will lay the foundation for the study of the development of the olfactory system.